Nginx + PHP-FPM Randomly Gives 502

Question

My site is working fine with Nginx + PHP-FPM but randomly it gives 502 error. Environment Details

OS - CetnOS 6
Nginx
PHP-FPM (php 5.4)
APC (Code Cache APC 3.1.13  beta)
Memcache (data cache)

In php-fpm

pm.max_children = 200
pm.start_servers = 40
pm.min_spare_servers = 30
pm.max_spare_servers = 50
pm.max_requests = 500

Also i am using TCP connection not socket.

If any body has any input please update me.

Thanks

Camel

nginxtips.com/502-bad-gateway-using-nginx – Trinitrophenol

Hi Alex,I tried tips mentioned on provided link disabling APC & set fcgi buffer and timeout as per link but did not got any success – Camel

Answer 1

Firstly reduce pm.max_children = 200 to pm.max_children = 50.

You will have to firstly increase the file limit of the system thereby allowing nginx and php-fpm to open a larger number of files. File limit has to be increased as in linux everything in the end is a file. So the more connections you open the more number of files will be required. In ubuntu the file limit configuration are done in /etc/security/limits.conf. You will need to locate this for CentOS.

Then try to increase the internal port range which can be used by php-fpm. Along with this tcp ports are generally associated with a timeout value before they are reused, reduce this timeout so that more ports are freed when their job is done.

Find detailed info here.

Addition:

In case if the error still persists try increasing the number of php-fpm worker processes to 100. Although it is not recommended to set the value so high as they consume addition memory.

pm.max_children = 100
pm.start_servers = 90
pm.min_spare_servers = 70
pm.max_spare_servers = 100

You can try out various values to get the optimum suited for your purpose.

The basic reason for a 502 is when nginx cannot forward or fails to forward the request to php-fpm. Increasing the number of php-fpm worker processes can be one of the way, thereby giving nginx more processes to forward the requests to.

Unquestioning

Hi , I have reduced number of pm.max_children to 50 and increased hard and soft file limits and modified /etc/sysctl.conf as per link provide. But i am still getting 502 errors randomly. – Camel

@user524694 What is the requests per second number you are testing at? – Unquestioning

Hi , I am getting following error "122252#0: *31344372 upstream prematurely closed FastCGI stdout while reading response header from upstream," and number of requests even low means 150 active connections and 30 writing and 120 reading it still gives this error with 4 PHP backend servers. – Camel